Accumulation of volatile phenol glycoconjugates in grapes after bushfires

This study demonstrated the different effects of the same amount of smoke on different grape varieties.

Introduction

It is well-documented that wines made from grapes that were exposed to smoke from bushfires can contain smoky and ashy characteristics.1 These characteristics are derived from volatile phenols, such as guaiacols, cresols and syringols, which are taken up by the grapevine during a smoke event and stored in the fruit, mainly as glycoconjugates (bound to sugars). Both free volatile phenols, as well as their glycoconjugates, can contribute to the negative sensory experience a wine drinker has when drinking wine from smoke-exposed grapes. The reason for this is because enzymes from bacteria in a person’s mouth can release the sugars from the glycosylated phenols, rendering them volatile2 and able to impart smoke flavour. It is therefore possible to detect very little or no smoke taint in the aroma of a wine, yet on the palate the wine can be perceived as ashy. Individuals also differ in terms of sensitivity towards tasting smoke taint.

Experimental layout

In the study researchers exposed Sauvignon blanc, Chardonnay and Merlot grapevines to smoke for one hour, approximately 10 days after veraison, using purpose-built smoke tents.3 Three treatments in total were conducted: a control treatment with no kaolin and no smoke, a smoke treatment, and a treatment of kaolin addition 24 hours before smoke treatment. Samples were collected at one day and seven days after smoke treatment, as well as at maturity, which were 15, 12 and 30 days after smoke exposure for Sauvignon blanc, Chardonnay and Merlot respectively. Volatile phenol concentrations were determined by GC-MS and phenol-glycoconjugate concentrations were determined by LC-MS/MS. Reflectance measurements, using a “Jaz” spectrometer, were performed one day prior to smoke exposure, as well as one and seven days after smoke exposure.

Results

In this study the application of kaolin and/or smoke had no impact on ripening and berry development. Smoke exposure did not yield grapes with significant levels of free volatile phenols at one or seven days after smoke exposure. None of the volatile phenols measured, were detected in control grapes and only very low levels in the exposed grapes. This finding was quite surprising. In contrast, significant differences were found in the glycoconjugate concentrations between control and exposed grapes, as well as between grape varieties. Glycoconjugate compositional differences (different sugar combinations) were also observed between varieties and time points that included measurements at maturity. The researchers suggested from these results that glycosylation occurs very soon after smoke exposure, hence the near absence of free volatiles one day after exposure.

Overall the glycoconjugate concentrations were much higher in Merlot grapes in comparison to Sauvignon blanc and Chardonnay that had received the same amount of smoke. The researchers postulate that the observed varietal differences can be ascribed to differences in berry physiology: skin thickness and the presence (or not) of cuticular waxes. The compositional differences in glycoconjugates observed between varieties could also potentially be ascribed to a variation in enzymes responsible for glycosylation.

The study presented an interesting anomaly: the low volatile and glycoconjugated phenol concentrations one day after smoke exposure did not explain the increase in concentrations observed during further ripening. This finding suggests that intermediate forms of the “smoke taint” phenols exist for a short period of time that are not detectable by the current GC-MS and LC-MS/MS methods.

The kaolin treatment had no effect on the uptake of volatile phenols and glycoconjugate formation in the case of Sauvignon blanc, and only a moderate effect on the uptake of some phenols in the case of Chardonnay. Kaolin applied as an aqueous suspension (mixed with water) to fruit and foliage before smoke treatment did have a significant effect on the uptake of volatile phenols in the case of Merlot. Reductions of 58 – 92% of most volatile phenol glycoconjugates measured at maturity were observed.

Spectral reflectance measurements were only successful differentiating between control and smoke-exposed grapes in the case of Chardonnay at one and seven days and Sauvignon blanc at one day after exposure, but not at seven days.

Significance of the study

Merlot have had the highest glycoconjugated phenol concentrations after smoke treatment compared to Sauvignon blanc and Chardonnay. Kaolin application to foliage and grapes can potentially mitigate volatile phenol uptake, as demonstrated in the case of Merlot. Spectral reflectance measurements have potential, but further research is needed.

Abstract

In a recently published study (January 2019) Australian researchers investigated the accumulation of volatile phenol glycoconjugates in different grape varieties following exposure to smoke ten days after veraison.3 In addition they investigated possible mitigation of smoke taint by foliar application of kaolin, as well as whether they could detect smoke-tainted grapes in the vineyard using a handheld spectrometer.

The study found significant differences in glycoconjugate concentrations between grape varieties, as well as within the same grape variety, depending on the time of measurement. Kaolin was able to significantly reduce glycoconjugate concentrations in only one grape variety and the differentiating between control and smoke-tainted grapes using a handheld spectrometer was also effective for one grape variety only.